A conventional fuel cell stack 1, shown in FIG. 3, has two or more individual fuel cells 2 which are stacked above one another in the manner of a tower. Each fuel cell 2 has an electrolyte layer 3, a cathode layer 4 arranged on one flat side of the electrolyte layer 3, and an anode layer 5 arranged on the other flat side of the electrolyte layer 3. For contacting a neighboring fuel cell 2, a contacting layer 6 is disposed on the cathode layer 4.
In addition, each individual fuel cell 2 has first and second separator plates 7, 8 that bound a combustible-gas space 9, into which the anode layer 5 projects. The combustible-gas space 9 is connected with the anode layer 5 such that combustible gas, which flows through the combustible-gas space 9 (direction of the arrow 10), can come in contact with the free surface of the anode layer 5.
Between the second separator plate 8 of one fuel cell 2 and a first separator plate 7 of the neighboring fuel cell 2, an oxidation gas space 11 is constructed, through which oxidation gas can flow (direction of the arrow 12), so that oxidation gas can flow against the free surface of the cathode layer 4, which projects into the oxidation gas space 11.
One flat side of the contacting layer 6 is in contact with the cathode layer 4 while the other flat side contacts a flat side of a first separator plate 7 of the neighboring individual fuel cell 2 (the latter facing the oxidation gas space 11). By way of corresponding openings 13 in the first and second separator plates 7 and 8, all combustible-gas spaces 9 are connected with one another. In the area between a second separator plate 8 and a first separator plate 7 of a neighboring individual fuel cell 2, the combustible-gas spaces 9 are separated in a gas-tight manner from the oxidation gas space 11 by means of a sealing layer 14, so that a fuel feeding duct 15 and a removal duct 16 for the reaction products are formed. Thus, combustible gas can be fed to the combustible-gas spaces 9 in the direction of the arrow 18 and flows through these in the direction of the arrow 10. In this case, the combustible gas is oxidized in a fuel cell 2 along the anode layer 5, and the reaction product can leave the fuel cell stack 1 again in the direction of the arrow 19. By way of correspondingly constructed feeding and removal ducts, the oxidation gas, analogous to the combustible gas, is guided through the oxidation gas spaces 11.
The separator plates 7 and 8 of an above-described fuel cell stack 1 therefore, on the one hand, have the function of electrically connecting the individual fuel cells 2, which are disposed in series. On the other hand, they ensure the separation of combustible and oxidation gas. For this purpose, the separator plates 7 and 8 (also called bipolar plates or interconnector plates) are constructed of a combustible-gas-tight, oxidation-gas-tight and electronically conductive material, such as a chrome-containing alloy, ferritic steel or perovskite. In order to ensure a reliable separation of the oxidation gases and the combustible gases, it is required that, in each case, between the second separator plate 8 of a first fuel cell 2 and the first separator plate 7 of a neighboring fuel cell 2, the feeding duct 15 as well as the product removal duct 16 be reliably sealed off from the oxidation gas space 11.
It is known from the state of the art to construct the sealing layer 14, for example, of glass-ceramic solders, which are normally applied as pastes or etched foils before the assembling of a fuel cell stack 1 onto the relevant sealing surfaces of the separator plates 7, 8.
These sealing materials (glass-ceramic solders) normally used in the case of solid-electrolyte fuel cells have two characteristics influencing one another in opposite directions. The coefficient of thermal expansion of the sealing material is clearly lower in comparison to the coefficients of expansion of most materials used for the bipolar plates 7 and 8. During the rapid heating of the fuel cell stack 1, this may result in thermally induced tension cracks in the sealing layer 14 and thus in a failure of its sealing effect. This is particularly critical in the case of solid-electrolyte fuel cells (the so-called SOFC's—solid oxide fuel cells) which operate in the high-temperature range. Particularly for solid-electrolyte fuel cells, which are stressed by a frequent starting and switching-off of the operation, this represents a problem which has not been satisfactorily solved.
From the state of the art, it is conventional to increase the coefficient of expansion of the sealing materials by means of additions. However, these additions frequently lead to a reduction of the electric resistance of the sealing material at the typically high operating temperatures of a solid-electrolyte fuel cell. By way of the sealing layer 14 between a second separator plate 8 and a first separator plate 7 of two neighboring individual fuel cells 2, this results. in undesirable leak currents which impair the electric efficiency of a fuel cell stack 1.
Another disadvantage of the sealing device known according to FIG. 3 of the state of the art is that the materials for the sealing layer 14 have a compression behavior and/or shrinking behavior which differs in comparison to the contacting layer 6, whereby, during the mounting of the fuel cell stack 1, undesirable inaccuracies occur which may make a reliable contacting between the contacting layer 6 and an adjoining separator plate 7 doubtful. Furthermore, it is disadvantageous that the providing of a suitable sealing layer 14 before the assembling of the fuel cell stack 1 requires high expenditures and cost because, for example, a sealing agent strand has to be established or, in the case of a foil-type construction of the sealing layer 14, the latter has to be produced separately and has to be positioned or inserted before the assembling process.
The above-mentioned glass-ceramic solders have two serious disadvantages:
1. The coefficient of thermal expansion of glass ceramics is clearly lower in comparison to the coefficients of expansion of most materials (chrome alloys, ferritic steel, perovskite) used for the bipolar plates. During the rapid heating of the fuel cell stack, this may result in thermally induced tension cracks in the sealings and thus in a failure of the sealing effect. This is particularly critical in the case of a mobile use of the fuel cell stack, for example, in an auxiliary energy supply unit in an automobile.
2. Glass-ceramic solders shrink during the joining process, that is, during the pressing-together and the first heating to the operating temperature of 750-900° C., to approximately 40%-70% of their initial volume. The entire stack therefore sinks together during the joining process. In order to ensure the tightness of the stack, the porous electric contacting layer 6 of the fuel cell (see FIG. 3) also has to shrink by the same thickness. The difficulty now consists of coordinating the shrinkage behavior of the sealing layer and of the contacting layer. The pasty ceramic suspensions normally used for the electric contacting shrink even at low temperatures and compact at temperatures higher than 400° C. In the case of glass-ceramic solders, the shrinking process starts only at temperatures >500° C. and is concluded only at temperatures >750° C. The two processes therefore do not take place simultaneously and frequently result in gas leakages, lack of electric contacting or a fracture of the SOFC (solid oxide fuel cell) because of locally excessive contact pressure forces.
Based on the above-mentioned disadvantages of the glass-ceramic solders, the development of an alternative inorganic sealing mass was carried out. With respect to its coefficient of expansion, it is better adapted to the used bipolar plate materials and has only minimal shrinkage during the joining process, so that the necessity of using electric contacting materials especially adapted in their shrinkage behavior is eliminated. However, the disadvantage of this sealing paste is an electric insulating capacity which is insufficient at the operating temperature. When solid-electrolyte fuel cell stacks are used, this results in electric leak currents (short circuits) between the individual bipolar plates and thus results in power losses in the system.
German Patent Document DE 19515457 C1 describes a sealing structure for a fuel cell. The fuel cell has an electrolyte layer that consists of an electrolyte matrix saturated with an electrolyte and, in the sealing area, the electrolyte matrix is constructed to be extended beyond the electrodes. In the sealing area, the electrolyte matrix is saturated by means of a material chemically related to the electrolyte, which material is firm at the working temperature of the fuel cell. However, the suggested solution relates to a so-called molten-carbonate fuel cell which has a molten electrolyte which is present in liquid form in an electrolyte matrix. In the case of this type of fuel cell, one usually speaks of a wet-sealing area because the electrolyte which is molten in its operating condition, forms a wet area in the edge region which is to be sealed off. However, this solution cannot be transferred to a solid-electrolyte fuel cell since, in the case of such a solid-electrolyte fuel cell (SOFC: solid-oxide fuel cell), no so-called wet electrodes or wet electrolytes exist, and thus the problem on which German Patent Document DE 19515457 C1 is based does not occur as a result of the type of construction.
German Patent Document DE 19960516 A1 describes a sealing device for a fuel cell. The fuel cell has an electrolyte membrane that is extended into the edge sealing area between two separator plates and a two-layer rubber seal that is arranged on the electrolyte membrane. For the sealing structure, it is suggested that one layer be constructed of a soft sponge rubber and the second layer be constructed of a harder rubber, such as silicone rubber or butyl rubber. This document relates to a so-called low-temperature fuel cell with a polymer membrane electrolyte. These so-called low-temperature fuel cells have operating temperatures which are in the range of between 60° C. and 80° C. Because of their operating temperatures, such fuel cells cannot be compared with a solid-electrolyte fuel cell because normally solid-electrolyte fuel cells are operated in temperatures range of between 700 and 1,100° C. Because of the high operating temperatures of a solid-electrolyte fuel cell, the sealing device suggested in German Patent Document DE 19960516 A1 can therefore not be transferred to a solid-electrolyte fuel cell.
Japanese Patent Document JP 10092450 shows a fuel cell stack with insulating layers and sealing layers, arranged in layers and formed as separate components.